An apparatus providing Fault Detection Diagnostics (FDD) for a Heating, Ventilating, Air Conditioning (HVAC) system comprising a permanent magnet attached to a movable damper and a magnetometer attached to a stationary frame to provide a magnetic field measurement of the permanent magnet. The apparatus converts the magnetic field measurement into a damper position measurement and determines a difference between the damper position measurement and a damper position actuator voltage command, and if the difference is greater than a damper actuator voltage tolerance, then the apparatus generates a FDD alarm signal. The apparatus calculates a com puted Outdoor Air Fraction (OAF) damper position based on a measured HVAC parameter, and if the difference between the computed OAF damper position and the OAF damper position command is greater than a damper position tolerance, then the apparatus generates a FDD alarm signal or an actuator voltage signal to correct the movable damper position.